Sulfate scale dissolution

ABSTRACT

Alkaline earth metal scales, especially barium sulfate scale deposits are removed from oilfield pipe and other tubular goods with a scale-removing composition comprising an aqueous alkaline solution having a pH of about 8 to about 14, preferably about 11 to 13, of a polyaminopolycarboxylic acid, preferably EDTA or DTPA and a catalyst or synergist comprising thiosulfate or nitriloacetate anions. When the scale-removing solution is contacted with a surface containing a scale deposit, substantially more scale is dissolved at a faster rate than is possible without the synergist.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of copending application Ser. No. 484,970, filed Feb. 26, 1990, which is a continuation part of prior co-pending application Serial No.07/332,147, filed Apr. 3, 1989, of J. M. Paul and R. L. Morris, now abandoned.

This application is also related to prior application Ser. Nos. 07/369,897, filed June 22, 1989and 07/431,114, filed Nov. 3, 1989, of the same inventors.

FIELD OF THE INVENTION

This invention relates to compositions which are effective for solubilizing and removing scale, particularly strontium and barium sulfate scale, from surfaces with scale deposits on them. It is particularly useful for the removal of such scale from oilfield equipment including downhole pipe, tubing and casing as well as subterranean formations. It is also applicable to the removal of these scale deposits from other equipment such as boilers and heat exchangers.

BACKGROUND OF THE INVENTION

Many waters contain alkaline earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulfate, bicarbonate carbonate, phosphate, and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when the concentrations of the barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. Solubility products are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.

As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.

Barium and strontium sulfate scale deposits present a unique and particularly intractable problem. Under most conditions, these sulfates are considerably less soluble in all solvents than any of the other commonly encountered scale-forming compounds, as shown by the comparative solubilities given in Table 1 below.

                  TABLE 1                                                          ______________________________________                                         Comparative Solubilities, 25° C. in Water.                              Scale           Solubility, mg./l.                                             ______________________________________                                         Gypsum          2080.0                                                         Strontium sulfate                                                                               140.0                                                         Calcium Carbonate                                                                               14.0                                                          Barium sulfate    2.3                                                          ______________________________________                                    

It is generally acknowledged that barium sulfate scale is extremely difficult to remove chemically, especially within reasonably short periods of time: the solvents which have been found to work generally take a long time to reach an equilibrium concentration of dissolved barium sulfate, which itself is usually of a relatively low order. Consequently, barium sulfate must be removed mechanically or the equipment, e.g. pipes, etc., containing the deposit must be discarded.

The incidence of barium sulfate scale is worldwide, and it occurs principally in systems handling subsurface waters. Because of this, the barium sulfate scale problem is of particular concern to the petroleum industry as water is generally produced with petroleum and as time goes on, more petroleum is produced by the waterflooding method of secondary recovery, implying even greater volumes of produced water. The scale may occur in many different places, including production tubing, well bore perforations, the area near the well bore, gathering lines, meters, valves and in other production equipment. Barium sulfate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well.

Barium sulfate scale is particularly troublesome when sulphate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. This particular aspect of the barium scale problem is severe in some U.S. oil fields as well as some older North Sea oil fields. Scaling of this nature is also expected to occur during advanced production stages in other North Sea fields particularly after seawater breakthrough has taken place.

Another problem associated with the formation of barium and strontium sulfate scales is that radium, another member of the alkaline earth group of metals tends to be deposited at the same time so that the equipment becomes radioactive, and may eventually have to become unusable for safety reasons alone. At present, a considerable amount of oilfield tubular goods are in this condition and cannot be readily restored to usable condition because of the difficulty of removing the radioactive scale.

Various proposals have been made in the past for removing barium sulfate scale chemically. Most of these processes have utilised chelating or complexing agents, principally the polyaminopolycarboxylic acids such as ethylenediaminetetraacetic acid(EDTA) or diethylenetriaminepentaacetic acid(DTPA).

U.S. Pat. No. 2,877,848(Case) discloses the use of EDTA in combination with various surfactants for this purpose. U.S. Pat. No. 3,660,287 (Quattrini) discloses the use of EDTA and DTPA in the presence of carbonate ion at relatively neutral pH (6.5-9.5) and U.S. Pat. No. 4,708,805(D'Muhala) discloses a process for the removal of barium sulfate scale by sequestration using an aqueous solution of citric acid, a polycarboxylic acid such as carbazic acid, and an alkylene-polyaminopolycarboxylic acid such as EDTA or DTPA. The preferred aqueous sequestering solutions have a pH in the range of about 9.5 to about 14, provided by a base such as potassium hydroxide or potassium carbonate.

Another approach which has recently been made is to use a polyether in combination with the aminopolycarboxylic acid. U.S. Pat. No. 4,190,462(deJong) discloses that barium sulfate scale can be removed from remote locations extending into a subterranean earth formation by contacting the scale with an aqueous solution consisting essentially of water, a monovalent cation salt of a monocyclic macroyclic polyether containing at least two nitrogen-linked carboxymethyl groups and enough monovalent basic compound to provide a solution pH of about 8. Similar disclosures are to be found in U.S. Pat. Nos. 4,215,000 and 4,288,333. These polyether materials have, however, the disadvantage of being costly which is a severe drawback for oilfield use where cost is a major factor.

Although many of these known compositions will remove scale, the rate of dissolution is slow and the amount of scale dissolved is small.

SUMMARY OF THE INVENTION

We have now found a way of removing barium sulfate scale using various novel combinations of scale-removing agents. These combinations are capable of removing scale at markedly higher speeds than prior scale-removing compositions and are also capable of removing relatively more scale for a given quantity of solvent. They are, moreover, relatively cheap and are therefore well suited to use in oilfield operations.

According to the present invention, barium sulfate and other sulfate scales are removed by a chemical process using a combination of a polyaminopolycarboxylic acid such as EDTA or DTPA together with a thiosulfate or nitriloacetate anion as a synergist or catalyst for the dissolution. The scale is removed under alkaline conditions, preferably at pH values of at least 10, usually 10-14, with best results being achieved at about pH 12.

The concentration of synergist or catalyst is usually about 0.01M to about 1.0M, preferably about 0.5M, with similar concentrations being appropriate for the primary chelant (the polyaminopolycarboxylic acid). Substantially improved scale dissolution rates are obtained when the aqueous solution containing the composition is at a temperature of about 25° C. to about 100° C. but higher temperatures are obtainable downhole because at greater formation depths higher existing pressures will raise the boiling point of the aqueous solution, and consequently greater scale removal rates may be attained.

The composition is particularly useful for more efficiently removing barium or strontium sulfate scale from wells, wellstream processing equipment, pipelines and tubular goods used to produce oil from a subterranean formation.

THE DRAWINGS

In the accompanying drawings:

The single figure is a graph which shows the rate of dissolution of barium sulfate in solutions of DTPA containing thiosulfate or nitriloacetate anion as a synergist,

DETAILED DESCRIPTION

According to the present invention, alkaline earth metal sulfate scales, especially barium sulfate scale, are removed by the use of a combination of chemical scale-removing agents. The method is particularly useful for the removal of such scale from oilfield equipment used to bring oil and/or water from subterranean formations to the surface. The method may, however, also be used to remove scale from the formations themselves, especially in the regions surrounding production and injection wells, as mentioned above. The method may also be used to remove scale from above- ground equipment both in the oilfield and elsewhere, for example, from boilers and heat exchangers and other equipment exposed to scale-forming conditions.

The scale itself is usually in the form of an adherent deposit of the scale-forming mineral on metal surfaces which have been exposed to the water containing the scale-forming components. These components comprise alkaline earth metals including calcium, strontium and barium, together with variable amounts of radium, depending upon the origin of the waters. As noted above, barium sulfate scale is particularly difficult to remove by existing chemical methods in view of its very low solubility.

The present scale removal is effected with an aqueous solvent which comprises a polyaminopolycarboxylic acid such as EDTA or DTPA as a chelant or chelating agent which is intended to form a stable complex with the cation of the alkaline earth scale-forming material. Of these chelants, DTPA is the preferred species since it forms the most soluble complexes at greatest reaction rate. EDTA may be used but is somewhat less favorable and, as noted below, may be less responsive to the addition of the catalyst or synergist. The chelant may be added to the solvent in the acid form or, alternatively, as a salt of the acid, preferably the potassium salt. In any event the alkaline conditions used in the scale removal process will convert the free acid to the salt.

The concentration of the chelant in the solvent should normally be at least 0.1M in order to achieve acceptable degree of scale removal. Chelant concentrations in excess of 1.0M are usually not necessary and concentrations from about 0.3M up to about 0.6M will normally give good results; although higher concentrations of chelant may be used, there is generally no advantage to doing so because the efficiency of the chelant utilisation will be lower at excess chelant concentrations. This economic penalty is particularly notable in oilfield operations where large volumes of solvent may be used, especially in formation scale removal treatment.

In addition to the chelant, the present scale removal compositions contain a catalyst or synergist for the dissolution of the scale. This catalyst is a thiosulfate or nitriloacetate anion. Thiosulfate salts, especially sodium thiosulfate or potassium thiosulfate provide a suitable source of the thiosulfate anion since the acid itself is unstable; these salts are, moreover, readily available commercially at reasonable cost. Nitriloacetate anions may be supplied by the acid itself, which is converted to the salts under the pH conditions employed, or by the addition of a nitriloacetate salt. Nitriloacetic acid, otherwise known as triglycine, N(CH₂ COOH)₃, is an acid which is insoluble in water but which readily forms water soluble mono-, di-, and tribasic salts from various cations including the alkali metals, especially sodium and potassium. The addition of the free acid to the solvent will therefore result in dissolution under the pH conditions prevailing. Alternatively, the nitriloacetate anions may be added by addition of the nitriloacetate salts, especially the sodium or potassium salts. In either case, the alkaline conditions will result in the formation of the tribasic nitriloacetate anion, N(CH₂ C00)₃ ³⁻, in the solvent. The potassium salts are preferred in view of their greater solubility and for this reason, the solvent should preferably be brought to the desired pH value with a potassium base, preferably potassium hydroxide.

The concentration of the catalyst or synergist in the aqueous solvent will be of a similar order to that of the chelant: thus, the amount of the synergist anion in the solvent should normally be at least 0.1M in order to achieve a perceptible increase in the efficiency of the scale removal, and concentrations from about 0.3M up to about 0.6M will give good results. Although higher concentrations of the synergist e.g. above 1.0M may be used, there is generally no advantage to doing so because the efficiency of the process will be lower at excess catalyst concentrations. Again, this economic penalty is particularly notable in oilfield operations.

The scale removal is effected under alkaline conditions preferably at pH values of from about 8.0 to about 14.0, with optimum values being from about 11 to 13, preferably about 12.

The prefered solvents comprise about 0.1 to about 1.0M of ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA), or salts of these acids, as a chelant. In addition, the thiosulfate or nitriloacetate anion as the catalyst or synergist is added to the aqueous solution in about 0.01 to about 1.0, preferably about up to 0.5M. The pH of the solvent is then adjusted by the addition of a base, preferably potassium hydroxide, to the desired value, preferably to about pH 12. We have found that it is important to avoid the use of sodium cations when operating at high pH values, above pH 8, and instead, to use potassium or, alternatively, cesium as the cation of the scale-removing agent. Potassium is preferred for economy as well as availability. Thus, the normal course of making up the solvent will be to dissolve the chelant and the potassium salt of the selected synergist in the water to the desired concentration, after which a potassium base, usually potassium hydroxide is added to bring the pH to the desired value of about 12. This aqueous composition can be used to remove scale from the equipment, or alternatively, pumped into the subterranean formation when it is the formation which is to be subjected to descaling.

The mode of operation of the synergist or catalyst is not presently understood. While not desiring to be bound to a particular theory concerning the actual mechanism of its activity in converting or dissolving the scale, it is believed that adsorption of the synergist or catalyst on the barium sulfate surface may modify the surface crystal structure in such a way that the barium in the modified crystal is easily removed by the chelating agent.

The aqueous solution containing the composition can be directed down a wellbore to remove barium sulfate scale which has fouled the tubular equipment e.g. piping, casing etc., and passage ways. Prior to being directed into the wellbore, the composition may be heated to a temperature between about 25° C. to about 100° C., although the temperatures prevailing downhole may make pre-heating unnecessary. Once within the tubular goods and the passageways requiring treatment, the composition is allowed to remain there for about ten minutes to about 7 hours. After remaining in contact with the equipment for the desired time, the composition containing the dissolved scale is produced to the surface and may be disposed of as required, possibly by re-injection into the subsurface formation. This procedure can be repeated as often as required to remove scale from the equipment.

In one procedure for circulating the solvent through the tubular goods in the well the solvent is pumped down through the production tube and returned to the surface through the annular space between the production tubes and the casing (or vice versa). Also, the cleaning solution may be pumped down through the production tubing and into the formation, thereby cleaning the well, including the well casing, and the formation pore space by dissolving barium sulfate present as it flows over and along the surfaces that need cleaning. The spent composition containing the dissolved, complexed barium together with any other alkaline earth metal cations which may have been present in the scale, especially radium, can be subsequently returned to the surface, for example, by displacement or entrainment with the fluids that are produced through the well after the cleaning operation. In an alternative manner, the cleaning solution may be applied batchwise fashion, for example, by flowing the solution into the well and optionally into the pore spaces of the adjacent earth formation and there keeping the solution in contact in non-flowing condition with the surfaces that are covered with barium sulfate scale, for a period of time sufficient to dissolve the scale.

The present scale removal technique is very effective for lowering residual radioactivity of pipe contaminated with radium-containing barium sulfate scale. As noted above, radium is frequently precipitated with barium in scale with the result that scaled pipe is often radioactive to the point that it cannot safely be used. Using the present scale removal compositions, activity can be reduced to an acceptable levels comparatively short times without further treatment. Some residual activity which arises from lead and other radio-isotopes which are not dissolved in the solvent: these isotopes are decay products of radium and have originally been incorporated in the scale with the barium and the radium sulfates. Although they are not removed chemically by the present scale removal technique, the dissolution of the barium scale together with the other alkaline earth metal sulfates enables these other components of the scale to be removed by simple abrasion, for example, by scrubbing with or without a detergent/water scrub solution. In this way, the residual activity level may be reduced to a very low value, below the appropriate regulatory standards. Thus, by using the present chemical scale removal technique in combination with a simple mechanical removal of loose, non-adherent material, previously radioactive pipe may quickly and readily be restored to useful, safe condition.

In order to demonstrate the barium sulfate scale-dissolving capacities of the composition, several aqueous solutions have been tested in laboratory tests the results of which are described in the discussions which follow. The experiments described below were, except as noted below, carried out in a cylindrical glass vessel having a height of 10 cm and an internal diameter of 7.5 cm. Barium sulfate as a scale scale component was added to the test solution and agitated with the selected solvents and the rates of dissolution and final dissolved concentrations determined. The solvents used comprised 0.5M DTPA in combination with either 0.5M thiosulfate added as potassium thiosulfate K₂ S₂ O₃, or 0.1M nitriloacetate, added as nitriloacetic acid, with the required pH of 12 attained by the addition of potassium hydroxide. The results are reported graphically in the Figure.

As shown in the Figure, DTPA alone and DTPA with thiosulfate and nitriloacetate synergists were compared at 100° C. The results demonstrate that the DTPA/synergist combination complexes more barium sulfate than DTPA alone, and that thiosulfate is slightly more effective as a synergist than nitriloacetate. Although the nitriloacetate catalyst was initially less effective than the chelant alone, the final equilibrium concentration of the dissolved scale component became greater at contact times longer than about 100 minutes, so that there is a potential for scale removal with this synergist at extended contact times using this low proportion of the synergist.

Distilled water was used in these tests. Minor decreases in efficiency may be observed with tap water and more significant decreases with seawater, e.g. about 20 percent decrease with seawater. This is reasonably to be expected since seawater has interfering ions, e.g. calcium and magnesium. These interfering ions complex with the chelating agent, either DTPA or EDTA, and reduce the overall dissolving power. Additionally, it has been determined that halide ions have a negative effect on dissolving power as a function of the size of the halide ion. Dissolution rate is increased as the halide ion size is reduced and the charge density is increased, i.e. in the order of iodide, bromide, chloride and fluoride. Fluoride ion enhances the effect of EDTA-based solvents, but not DTPA: fluoride inhibits most DTPA/catalyst solvents.

As noted above, the effect of cations is also very important to the success of the scale solvent, especially when added with the sizable portion of caustic required to adjust the pH to 12. Dissolution is enhanced as the size of the cation is increased, i.e. lithium, sodium, potassium and cesium. Lithium and sodium hydroxides in the presence of EDTA, or DTPA, and catalysts are not soluble at a pH of 12, the optimum value. Cesium is too difficult to obtain, both in quantity and price. Therefore, potassium hydroxide, in the form of caustic potash, is the pH adjusting reagent of choice. 

We claim:
 1. A composition for dissolving an alkaline earth metal sulfate scale comprising:a) an aqueous solution having a pH of from about 8 to about 14; b) a chelating agent comprising a polyaminopolycarboxylic acid or a salt of such an acid; and c) a synergistic scale-dissolving quantity of thiosulfate anions present from 0.05M to 1.0M.
 2. The composition as recited in claim 1 where the chelating agent is DTPA.
 3. The composition as recited in claim 1 where the pH is obtained by the addition of potassium hydroxides.
 4. The composition as recited in claim 1 where the chelating agent is present in an amount of 0.1M to 1.0M.
 5. The composition as recited in claim 4 where the chelating agent is present in an amount of from 0.3M to 0.6M.
 6. The composition as recited in claim 1 where the thiosulfate anions are present in a concentration of from 0.05M to 0.6M.
 7. The composition as recited in claim 6 where the thiosulfate anions are present in a concentration of 0.1M to 0.6M.
 8. The composition as recited in claim 1 where the pH of the solution is from 10 to
 13. 9. The composition as recited in claim 8 where the pH of the solution is about
 12. 10. An aqueous composition for dissolving alkaline earth sulfate scale comprising an aqueous alkaline solution having a pH of from about 8 to about 14, from about 0.05M to about 1.0M of a chelating agent comprising a polyaminopolycarboxylic acid or a salt of such an acid, and from about 0.05M to about 1.0M of a synergist comprising thiosulfate anions to improve the scale removing characteristics of the composition.
 11. The composition as recited in claim 10 where the pH is obtained by the presence of a potassium base.
 12. The composition as recited in claim 10 where the chelating agent comprises DTPA.
 13. The composition as recited in claim 12 where the pH is adjusted to a value from about 11 to 13 with potassium hydroxide, the chelating agent comprises DTPA and the synergist anion is present in a concentration of from 0.3M to 0.6M
 14. A method for removing alkaline earth sulfate scale comprising contacting the scale with an aqueous solution having a pH of about 8 to about 14 and comprising a chelating agent comprising a polyaminopolycarboxylic acid or salt of such an acid, and a thiosulfate anion as synergist present from 0.25M to 0.1M to dissolve the scale.
 15. The method as recited in claim 14 where the chelating agent comprises DTPA.
 16. The method as recited in claim 14 where DTPA is present in a concentration of from 0.1M to 1.0M.
 17. The method as recited in claim 14 where the pH of the solution is from 11 to
 13. 18. The method as recited in claim 14 where the scale comprises barium sulfate scale.
 19. The method as recited in claim 14 where the pH of the solution is provided by the use of a potassium base.
 20. The method as recited in claim 19 where the potassium base is potassium hydroxide.
 21. A method for reducing the radioactivity of oilfield tubular goods contaminated with radioactive, radium-containing scale of deposited alkaline earth metal sulfates, comprising contacting the scale on the tubular goods with an aqueous composition for dissolving alkaline earth sulfate scale comprising an aqueous alkaline solution having a pH of from about 11 to about 14, from about 0.05M to about 1.0M of a chelating agent comprising a polyaminopolycarboxylic acid or a salt of such an acid, and from about 0.05M to about 1.0M of thiosulfate anions as a synergist to improve the scale removing characteristics of the composition, for a time sufficient to remove the scale from the tubular goods.
 22. The method as recited in claim 21 where the chelating agent comprises DTPA in a concentration of from 0.1M to 1.0M, the synergist comprises thiosulfate in a concentration of from 0.1M to 1.0M , and the pH is from 11 to 13 provided by potassium hydroxide. 